Electroplating is extensively used in the electronic industry to metallize ceramic and organic substrates. The metallization required for various applications depends upon the required functionality of the substrates. There are several methods of metallization processes such as evaporation, sputtering, thermal spraying, electrodeposition, and electroless deposition. Of these different processes, electrodeposition is one of the most commonly used techniques due to its simplicity and scalability to substrates of various size. Additional advantages such as low cost of operation make electrodeposition a preferred method for metallizing a substrate.
In the electroplating process, electrical contact to the work piece is made through various mechanisms depending upon the size of the work piece and the critically of the contact area. For example, the electrical contact may be made through a metallic drum formed of metallic mesh in which small pieces are tumbled during plating. This contact method works well if the parts are very small and if micro-scratches on the surface of the deposited metal do not create a problem. Electrical contact may also be provided through a temporary frame attached to the work piece. This method is preferred when there is ample space on the plating surface which is not being plated. This is true when there is a wide edge on the part that is not critical. In some cases, the electrical connections are made by soldering wires to the work piece. All of these methods require, however, that a large area on the surface of the parts is available to provide a contact surface. This large area must either be not functionally critical or not sensitive to micro-damage on the surface.
A current procedure available in the art for making electrical contact to the work piece or substrate during electroplating of high-density electronic packages is by using very small pins making a point contact to the seed layer. The electrical connection to the work piece is made by a point contact to the seed layer on the top surface of the substrate. The point contact is incorporated into an auxiliary electrode but electrically isolated from the electrode. The point contact is connected to the power supply by conducting wires. The work piece and the auxiliary electrode are supported by a polymer block. The auxiliary electrode is held in place by mechanical snap-in locks and the work piece is held in place by vacuum. The complete assembly is immersed in electroplating solution and current is applied to both the work piece and the auxiliary electrode to accomplish metallization of the substrate.
The procedures now available for making electrical contact during electroplating have several limitations and raise concerns in the manufacturing environment. These limitations and concerns include:
(1) The contact area on the top surface of the substrate required by conventional processes is in very short supply: it is consumed at the expense of surface area required for functionality. The contact area becomes functionally unusable. It is preferable to have no electrical contacts on the top surface so that all the surface area can be used for functionality (to satisfy package requirements).
(2) The ends of the point contacts or pins also become coated during the electroplating process and, hence, create variability in the area of the surface being electroplated. This variability, in turn, creates variability in the thickness of the deposited metal (for a given amount of electricity passed). The thickness and uniformity of the plated conductor metal film are extremely important to the electrical functionality of the package.
(3) The insulating coating on the point contacts or pins can become eroded during the plating processes now used. This erosion results in shorts between the substrate or work piece and the auxiliary electrode. These shorts can produce a non-uniform thickness of the metal conductors across the substrate or work piece as well as non-optimal microstructure of the deposited metal (powdery metal compared to dense metal film). Consequently, the point contacts require considerable maintenance in the manufacturing environment to ensure that they do not erode.
(4) The insulation on the electrical wires which connect the pins to the power supply may also become eroded. Once the insulation has eroded, wires which are exposed to the plating solution are also subject to being plated. The plated wires can cause shorts with the auxiliary electrode creating the associated shorting problems as described above.
(5) Loss of contact to the seed layer can occur because of the manner in which mechanical contact is provided between the substrate and the current source (the electrical power supply). Poor or intermittent contact causes incomplete plating which requires the substrates to be scrapped. Scrapped substrates represent a yield loss.
(6) With the top side point contact scheme, a thicker seed layer film is required as substrates get larger in size. As the seed layer increases in thickness, it becomes increasingly more difficult to define and to isolate the conductor features from one another.
(7) Contact areas tend to plate thicker than the remainder of the conductor metal. This irregular thickness in contact areas can create problems during subsequent processing of the substrates, such as coating the substrate with another film such as polyimide.
What is needed is an improved method and apparatus for providing electrical contact to a substrate or work piece which is being electroplated. A method and apparatus are needed that avoid the shortcomings described above. A method and apparatus for electroplating a film uniformly onto a surface, and for providing a contact scheme which does not include the above shortcomings, are especially needed.